1. Field of the Invention
The present invention relates in general to laminated cylindrical coils for producing magnetic fields and relates in particular to such apparatus wherein insulated bus bars are inset within grooves formed in the coils.
2. Description of Prior Developments
The manufacture and construction of a gradient coil assembly for use in magnetic resonance imaging (MRI) apparatus can be complicated and costly. A typical gradient coil assembly includes the main coil subassembly which includes three cylindrical main coils known as the x, y and z main coils and the shield coil subassembly which includes corresponding cylindrical shield coils known as the x, y and z shield coils. The entire gradient coil assembly is mounted within the bore of an MRI magnet for producing the accurately controlled intense magnetic fields required to produce MRI images.
The x and y gradient coils can be formed from flat panels which are bent into 90xc2x0 or 180xc2x0 arcs and assembled into cylindrical fabrications. Such panel constructions used for the x and y coils are known as xe2x80x9cthumb-printxe2x80x9d 90xc2x0 or 180xc2x0 section panel coils. Each panel includes an electrical conducting layer laminated to a backing layer of insulation which may be of a resin base formulation such as G-10 fiberglass.
In order to properly distribute electrical current to the x and y gradient coils, bus bars are inserted radially between the x and y coils and between y and z coils. This placement of the bus bars takes up space between the x and y gradient coils as well as between the y and z gradient coils. The result is an overall gradient coil assembly having a relatively large radial thickness due to the placement of the bus bars.
When the bus bars are initially mounted to a panel coil, a layer of resinous composite material is generally applied over the entire outer surface of the bus bars and panel coils to form a cylindrical sleeve around the bus bars and their underlying gradient coils. This layer of composite material must be allowed to harden and cure. The time required for curing slows down the magnetic coil manufacturing process.
After the layer of composite material cures, it must be carefully machined to provide a smooth accurate cylindrical surface beneath and within which the bus bars are embedded. The carefully machined surface on the composite resin material is required for maintaining an accurate coaxial alignment between the x, y and z coils. This accurate alignment is needed to produce accurate magnetic field gradients.
The next layer of gradient coil material is then mounted over the machined resinous composite layer. It can be appreciated that the machining of the composite material adds significant time and cost to the fabrication of the gradient coil assembly.
The conventional assembly of the bus bars between the x and y gradient coils and between the y and z gradient coils not only requires significant time and effort, it results in a large diameter coil assembly. This is a drawback, since the end result is an MRI magnet having a smaller central opening. Small openings are considered unfavorably by patients who may experience claustrophobia when positioned in such small openings.
Moreover, if the main gradient coil subassembly and the shield gradient coil subassembly can be made more radially compact, the gradient coil assembly becomes more magnetically efficient insofar as it requires less electrical current to produce the same magnetic field as a more radially enlarged design.
Accordingly, a need exists for a more radially compact gradient coil assembly for an MRI magnet. A further need exists for a gradient coil assembly which eliminates one or more of the time consuming manufacturing steps associated with the fabrication of conventional gradient coils. Yet another need exists for a gradient coil assembly which facilitates the mounting of bus bars between concentric cylindrical gradient coils in an MRI magnet assembly.
The present invention has been developed to fulfill the needs noted above and therefore has an object the provision of a radially compact gradient coil assembly for use in MRI apparatus.
A further object of the invention is the provision of a bus bar mounting arrangement for mounting bus bars between adjacent concentric coils, such as gradient coils used in MRI apparatus.
Yet another object of the invention is the provision of an improved method of assembling gradient coils such that one or more conventional manufacturing steps are eliminated.
Another object of the invention is the provision of locating pins for facilitating the accurate alignment of consecutive layers of magnet coils, both axially and circumferentially.
These and other objects are met in accordance with the present invention which is directed to a compact gradient coil assembly wherein grooves are machined within the coil backing insulation material to receive electrical bus bars. The bus bars are used to distribute electrical power to the magnetic field generating coils concentrically mounted around one another.
Axial, circumferential, helical or other shaped grooves are machined in the insulating backing material typically provided on the sheet-like panels used to form the coils., These grooves allow the bus bars to be inserted within or inlaid into these recesses cut into the backing material. In this manner, the bus bars do not radially project above or beyond the surface of the backing material but are instead enshrouded within the backing material as opposed to being completely embedded in an overlay of cured adhesive resin. This recessed mounting of the bus bars substantially flush with the surface of the panel backing layer of the overlying panel allows the next adjacent overlying gradient coil to be mounted substantially directly to the surface of the underlying coil without the need for accommodating a radially projecting bus bar of the type necessitated by prior coil assembly constructions.
A particular advantage of this type of bus bar and coil mounting assembly is the reduction in the overall radial thickness of the main coil and shield coil subassemblies. That is, prior designs simply positioned a bus bar on the conductive outer surface of the panel coil and then applied a cylindrical layer of adhesive resin over the bus bar and the underlying panel coil. This layer of resin required time to cure and added to the radial growth of the main and shield coil subassemblies.
Once the resin dried or cured, it had to be carefully machined, as noted above. It can be appreciated that by inserting a bus bar into a groove in the coil backing material, so that the bus bar does not radially project to any significant degree above or beyond the surface of the backing material, a supplemental layer of resin is not needed to form a continuous cylindrical support layer around the radially protruding bus bar and its underlying coil. Such a supplemental resin layer was required in prior panel constructions to provide a continuous cylindrical mounting surface around which the next cylindrical coil was mounted.
By eliminating the supplemental resin layer between adjacent coils, the assembly time of the coil assembly can be reduced. Moreover, the time required for curing the supplemental resin layer can also be eliminated. A significant additional advantage of eliminating the supplemental resin layer is the elimination of the machining step previously required to form an accurate cylindrical support surface on the cured supplemental resin material.
The bus bars may be specially adapted for use with the MRI coil assembly of the present invention by having a layer of insulation provided along the radially inner surface of each bus bar. In this manner, the bus bars are prevented from electrically shorting the current flowing in the coils upon which the bus bars are mounted. Bus bars insulated in accordance with the invention are particularly well suited to the simplified x and y coil construction noted above.
Another feature of the invention is the use of radily-extending positioning pins for axially and circumferentially positioning a y coil over an x coil during their assembly. Such pins facilitate accurate fabrication and alignment of flat panels as they are flexed into accurate segments to form a cylindrical coil.
These and other objects of the invention will become more readily apparent from the following detailed description, taken together with the accompanying drawings.